Materials for use in solar battery back sheets include fluororesin films such as polyvinyl fluoride (PVF) films, aluminum foil laminates, and aluminum or Si vapor deposited polyethylene terephthalate (PET) films, which are for improving weather resistance and gas barrier properties. Such back sheets protect, from water vapor, solar battery cells which are sensitive to humidity.
Among the above back sheet materials, fluororesin films are superior in terms of long-term durability. Therefore, demand for fluororesin films as back sheet materials has been increasing. Unfortunately, materials made of fluororesins, which have generally C—F bonds, have the problem that they have low surface energy and show water repellency/oil repellency and therefore are less adhesive.
Examples of techniques for improving the adhesiveness of such fluororesin films include plasma discharge treatment, corona discharge treatment, flame treatment, and blast treatment. These surface modification techniques introduce a hydrophilic functional group (such as a —COOH, —OH, SO3H, or SO2Fx group) onto the resin surface to improve the adhesiveness.
However, such treatment methods need a large-scale apparatus and therefore increase production cost. They also have the problem that the modified surface significantly degrade over time and therefore cannot maintain the bonding capability over a long term.
On the other hand, Patent Document 1 listed below discloses a surface modification method that comprises: selecting a synthetic or natural polymer material having a specific gravity of 1.6 or less and having no ether, carbonate, amide, or urethane bond; and bringing the synthetic or natural polymer material into contact with a mixed gas composed of fluorine gas and a type of gas containing oxygen element so that hydrophilicity is imparted to the material.
Unfortunately, this conventional technique is not applicable to resins with a specific gravity of about 1.6 to 2.2. This is because synthetic or natural polymer materials with a specific gravity of more than 1.6 have a well-developed crystal structure and therefore resist diffusion of fluorine gas so that the function of surface modification is not well developed.